I've been thinking about building myself a rackmount rig to replace an old 12" combo amp, and was wondering if it really makes any difference to use a something that's marketed as a "guitar" speaker cabinet... Why not just select a nice PA speaker that's ostensibly already built to faithfully reproduce whatever is pushed through it?

This is especially of interest to me because I'd like to be able to use this rig for electric guitar, acoustic guitar, and mandolin. Ideally I'd have a rackmount preamp for the electric guitar and use a Baggs DI direct into the power amp for the acoustic instruments, and get appropriate tones on each instrument through one speaker cabinet.

There is one situation where using a full range, full frequency reproduction system like a PA makes a lot of sense: that is if you're using modelling technology like a Pod or an Axe-Fx that include speaker cabinet simulation. In that case, the use of an FRFR amplification system is one of the best ways to enjoy your rig, loud.
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Ian C.Jun 6 '11 at 0:46

Yeah, that's a really good point, thanks. I have the option of using a SansAmp GT2 in the electric guitar section of the rig, which does have speaker cabinet emulation baked in.
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RwwLJun 6 '11 at 0:59

Cabinet emulation is good. If you go dry, definitely do that. It's an extra set of algorithms that can make you sound slightly more natural--which is important when playing electric but not so much with acoustic.
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JduvJun 6 '11 at 11:43

5 Answers
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There's absolutely nothing wrong with using a PA speaker cabinet, especially if you plan to play amplified acoustic instruments through the rig. It might even produce a better overall sound with these.

Guitar cabinets are designed for a very specific purpose - electric guitar amplification and thus have their construction optimised for this purpose. They will generally have a limited frequency response - in the range best suited to the electric guitar - and will genrally be more robust than the typical PA speaker in terms of handling level and square-shaped waveforms, such as distorted guitar. Like guitar amplifiers, guitar cabinets are generally designed with a specific sound in mind and will contribute to the overall tone. This actually makes them less-suited for general amplification jobs, since they tend to colour the sound too much.

The drawback of using a standard PA speaker for an electric guitar amplification system is that it does faithfully represent whatever is fed through it, thus losing the aforementioned speaker colouration that is an element of many guitar tones and producing a sound not unlike a DI-ed guitar, which is disliked by many guitarists. You should try out both options before choosing one or the other.

Limited frequency response is key, but note that it's got less to do with the design of the cabinet and much to do with the choice of speaker. You can find guitar-specific speakers in a range of frequency-limiting bands and they all sound markedly different.
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Ian C.Jun 6 '11 at 0:45

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Cabinet designs are important. Closed back sounds completely different than open back and partially closed back. This is pretty much right on though. Good answer.
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JduvJun 6 '11 at 3:36

It is good to note that the difference between an acoustic guitar and an electric guitar.

The acoustic guitar is the full instrument.

The electric guitar is only one half of an instrument, the electric guitar and the electric guitar amplifier is the full instrument.

The electric guitar signals are raw and needs to be processed by the electric guitar amplifier to get the best results. That is why there are many effect boxes that can be used to further process the raw electric guitar signals before they are being amplified.

An electric guitar signals is like a wild horse that needs to be tamed before it is safe to ride it. Although it may seems ironic, the electric guitar signals may also be made more "wild" but in a controlled manner (distortion, overdrive, etc). The waveform is being shaped but the wild swing in amplitudes and frequencies (desired and undesired) are being moderated before they are being sent to the listeners' ears.

If you need to use your electric guitar on a regular PA amplifier, it will be good to let the signal go through an equalizer or effect box so you can shape and control the signal levels.

For Guitar, I totally agree with using PA amplifiers and cabinets loaded with ProAudio speakers, provided you are using a high-end guitar processor such as Fractal Axe FX, Avid Eleven rack, Roland VG99, etc. Virtually all guitar amplifiers include coloration such as tone controls, distortion/overdrive, presence, reverb, that would need to be set flat, neutral, or OFF. These features become redundant and even unwanted.

Today's processors do it all, including dynamics, delay, pitch-shifting, you name it. The guitar amplifier becomes nothing more than a power amp, neutral in all respects. If every patch you command from your pedal has custom settings designed to fine-tune that sound, why add further color from your amp head and speakers?

Now, let's talk about speakers. The whole idea behind a "guitar speaker" is to reproduce the range of frequencies a guitar is capable of. In addition, a design that allows for a desirable "break-over" point, both resulting in a characteristic rock&roll tone, and tolerating being overdriven without damage. ProAudio reproduces almost exactly what you feed it at a broad range of volume levels. The average Celestion, Eminence or Fane speaker, etc. is rated at 75 to 200 watts RMS and is intended to be overdriven, while ProAudio (for a 12") would be best specified at around 500 watts, and should never be overdriven. Generally, they offer a usable frequency range of 40-4000hz. The Roland VG99 processor is capable of dropping you down a full octave. Although you might not use those patches very often it would be nice to have a system that can support frequencies low enough to enable you to hang with your bass player.

The bottom line is: If you must have a true "vintage" sound, go with analog stomp-boxes, tube amps and a good sealed cabinet designed to give you that "Marshall" sound.
Otherwise it's digital all the way!

I used a Dayton XO2W-3.5K crossover. Cost $35.00. Installed it in a passive American audio cabinet with existing 15 in. speaker and horn/driver, $35.00 craigs list. Replaced the 15 in. with an Altec 421A that I already had. There all over on Ebay. Replaced the driver with an Altec 802-D that I also had. It's night and day differance in sound. I just use a Modeling amp, the Vox VT20+ with an simple added 1/4 in. out jack. Of course you can EQ it any way you wish. I like the sound because it retained the tone I was looking for. It's a cheap way to go. Honestly, it sounded pretty darn good with the original hardware.